FIG. 1 illustrates an exemplary network structure of an Evolved Universal Mobile Telecommunications System (E-UMTS) as a mobile communications system to which the related art and the present invention are applied. The E-UMTS evolved from an existing Universal Telecommunications System (UMTS). Basic standardization of the E-UMTS is currently being developed by the Third Generation Partnership Project (3GPP). The E-UMTS may be called a Long Term Evolution (LTE) system.
An E-UMTS network may be divided into an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and a Core Network (CN). The E-UTRAN includes a mobile terminal (User Equipment; UE), a base station (eNode B), and an Access Gateway (AG) located at the end of the network to be connected to an external network. The AG may be divided into a portion for processing a user traffic and a portion for processing a control traffic. The AG portion for processing the user traffic and the AG portion for processing the control traffic are connected to each other via a new communication interface. One or more cells may exist in a single eNode B. An interface for transmitting the user traffic or the control traffic can be used between eNode Bs. Also, the CN may consist of an Access Gateway (AG), a node for user registration of UEs, and the like. An interface for discriminating between the E-UTRAN and the CN can be used.
Radio interface protocol layers between mobile terminal and network may be classified into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of a well-known interconnection system, such as an open system interconnection (OSI) reference model. Among these, a physical layer belonging to the first layer provides an information transfer service using a physical channel. A Radio Resource Control (RRC) layer positioned in the third layer serves to control radio resources between the mobile terminal and the network. Accordingly, the RRC layer allows an RRC message exchange between the mobile terminal and the network. The RRC layer may be positioned in both eNode B and AG, or positioned only in one of the eNode B or AG.
FIGS. 2 and 3 show radio interface protocol architecture between a terminal and E-UTRAN based on 3GPP radio access network standards. Particularly, FIG. 2 shows radio protocol architecture in a control plane, and FIG. 3 shows radio protocol architecture in a user plane.
The radio interface protocol in FIGS. 2 and 3 has horizontal layers comprising a physical layer, a data link layer and a network layer, and has vertical planes comprising a user plane for transmitting user traffic and a control plane for transmitting control signals. The protocol layers in FIGS. 2 and 3 can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an Open System Interconnection (OSI) standard model widely known in communications systems. Hereinafter, each layer in the radio protocol control plane in FIG. 2 and a radio protocol user plane in FIG. 3 will be described.
A first layer, as a physical layer, provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to its upper layer, called a Medium Access Control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel. Data is transferred via a physical channel between different physical layers, namely, between the physical layer of a transmitting side and the physical layer of a receiving side. The physical channel is modulated based on an Orthogonal Frequency Division Multiplexing (OFDM) technique, and utilizes time and frequency as radio resources.
The MAC layer located at the second layer provides a service to an upper layer, called a Radio Link Control (RLC) layer, via a logical channel. The RLC layer of the second layer supports reliable data transmissions. The function of the RLC layer may be implemented as a functional block in the MAC layer. In this case, the RLC layer may not exist. A Packet Data Convergence Protocol (PDCP) layer of the second layer, in the radio protocol user plane, is used to efficiently transmit IP packets, such as IPv4 or IPv6, on a radio interface with a relatively narrow bandwidth. For this purpose, the PDCP layer reduces the size of an IP packet header which is relatively great in size and includes unnecessary control information, namely, a function called header compression is performed.
A Radio Resource Control (RRC) layer located at the lowest portion of the third layer is only defined in the control plane. The RRC layer controls logical channels, transport channels and physical channels in relation to establishment, re-configuration and release of Radio Bearers (RBs). Here, the RB signifies a service provided by the second layer for data transmissions between the terminal and the E-UTRAN.
Downlink transport channels for transmitting data from a network to a mobile terminal may include a Broadcast Channel (BCH) for transmitting system information and a Downlink Shared Channel (SCH) for transmitting a user traffic or a control message. A traffic or control message of a downlink multicast or broadcast service may be transmitted via the downlink SCH or via a separate downlink Multicast Channel (MCH). Uplink transport channels for transmitting data from a mobile terminal to a network may include a Random Access Channel (RACH) for transmitting an initial control message and an uplink Shared Channel (SCH) for transmitting a user traffic or a control message.
Also, downlink physical channels for transmitting information transferred to a downlink transport channel via an interface between a network and a mobile terminal may include a Physical Broadcast Channel (PBCH) for transmitting BCH information, a Physical Multicast Channel (PMCH) for transmitting MCH information, a Physical Downlink Shared Channel (PDSCH) for transmitting PCH information and downlink SCH information, and a Physical Downlink Control Channel (PDCCH) (or called DL L1/L2 control channel) for transmitting control information sent from the first and second layers, such as downlink or uplink radio resource allocation information (DL/UL Scheduling Grant) or the like. Uplink physical channels for transmitting information transferred to an uplink transport channel via an interface between a network and a mobile terminal may include a Physical Uplink Shared Channel (PUSCH) for transmitting uplink SCH information, a Physical Random Access Channel (PRACH) for transmitting RACH information, and a Physical Uplink Control Channel (PUCCH) for transmitting control information sent from the first and second layers, such as HARQ ACK or NACK, Scheduling Request (SR), Channel Quality Indicator (CQI) report and the like.
FIG. 4 is an exemplary view showing a random access procedure according to the related art.
As illustrated in FIG. 4, the mobile terminal firstly transmits a random access preamble to a base station via a selected PRACH resource using system information received from the base station. After receiving the random access preamble from the mobile terminal, then the base station transmits a random access response. The random access response information may include offset information (i.e., time advance value) for compensating a time synchronization of the mobile terminal, uplink radio resource allocation information (i.e., UL Grant) for a scheduled transmission, an index information (i.e., preamble Id) of a random access preamble received for identification between terminals performing a random access, a temporary identifier (i.e., Temporary C-RNTI) of the mobile terminal and the like. After receiving the random access response, the mobile terminal compensates a time synchronization according to the random access response information, and then transmits data including a terminal identifier (e.g., C-RNTI, S-TMSI or Random Id) to the base station using the uplink radio resource allocation information. The base station having received the data transmits a contention resolution to the mobile terminal using the terminal identifier transmitted from the mobile terminal.
Now, transmission and reception of random access preamble and random access response during the random access procedure will be described in detail. The mobile terminal receives system information required for the random access procedure from the base station. The system information may include PRACH radio resource information, an amount of transmission power of a first random access preamble, an increased amount of preamble transmission power, and the like. In particular, the PRACH radio resource information may consist of, for example, time/frequency information related to a radio resource by which the mobile terminal can transmit a random access preamble, information on a repetition period of the radio resource, and the like. That is, the mobile terminal can select the PRACH radio resource using the PRACH radio resource information, thus to transmit a random access preamble to the base station. One RA (Random Access)-RNTI is mapped to each PRACH radio resource. Here, the RA-RNTI denotes a random access identifier which is transmitted by being included in a control channel (i.e., PDCCH) required to receive the random access response transmitted from the mobile terminal via the DL-SCH. That is, the mobile terminal has transmitted the random access preamble by selecting the PRACH radio resource, whereas the mobile terminal tries to receive the PDCCH using the RA-RNTI included in the PRACH radio resource. Upon successfully receiving the PDCCH including the RA-RNTI, the mobile terminal can receive the random access response transmitted via the DL-SCH according to the PDCCH information. However, since the mobile terminal cannot continuously try to receive the PDCCH including the RA-RNTI after transmitting the random access preamble, the receiving window of the random access response information. For example, a time of 3 to 4 ms after transmitting the random access preamble is set as a receiving window of the random access response information. Accordingly, the mobile terminal tries to receive the PDCCH including the RA-RNTI within the receiving window. If the PDCCH including the RA-RNTI is not transmitted within the receiving window, the mobile terminal retries to transmit a random access preamble.
As aforementioned, during a random access procedure, the mobile terminal transmits a random access preamble, determines a RA-RNTI using a PRACH radio resource by which the random access preamble is transmitted, and tries to receive a PDCCH including the RA-RNTI within a random access response receiving window. If the PDCCH is failed to be received within the receiving window, the random access preamble transmission is retried.
On the other hand, in order to minimize an interruption occurred due to random access preambles with adjacent cells, the base station sets such that a random access preamble is transmitted at low power when the mobile terminal transmits a first random access preamble during the random access procedure. However, if the base station does not appropriately receive the random access preamble due to the low power, the base station cannot transmit a random access response, which causes the mobile terminal to fail to receive the PDCCH including the RA-RNTI within the random access response receiving window. Here, the mobile terminal increases the preamble transmission power step by step to retransmit a random access preamble to the base station. That is, the step-by-step increase in the transmission power of the random access preamble allows the base station to minimize the interruption to the adjacent cells, whereby the base station can appropriately receive a random access preamble.
However, in the related art, for the step-by-step increase in the transmission power of the random access preamble, the mobile terminal always transmits the random access preamble and then tries to receive the PDCCH transmitted with the RA-RNTI until the end of the random access response receiving window for the preamble. If failed, the mobile terminal should increase the amount of transmission power again to retransmit a random access preamble. That is, in order for the mobile terminal to reach an appropriate transmission power of the random access preamble, the mobile terminal should always wait for the random access response receiving window and thereafter repeat the transmission of the random access preamble, which causes unnecessary time delay during the random access procedure.